Ever since its first release candidate (Firefox RC 1), I have been a big fan of the new improvements in Firefox.
Some of the cool features;
- Drag and drop reordering for opened tabs.
- Non-obstructive error message (You can do that with firetune for pre-1.5 versions).
- Easier options for the security settings.
- Force links that open in New Window to open in a new Tab instead.
Firefox 1.5 Articles, Reviews
My personal Firefox 1.5 extension choice